Tomography is an analog imaging process which is widely used for radiographic imaging. In tomography, an x-ray beam source and an x-ray film are moved in predetermined directions relative to one another, such that the fulcrum of relative movement between the source and the film defines a region of interest. Since relative movement at the fulcrum is zero, this region is projected unchanged onto the film. The radiographic projections of all tissues lying outside this region move during exposure, so that they appear more or less blurred depending on their distance from the fulcrum. The amount of blur is directly proportional to their distance from the fulcrum. Accordingly, the region or fulcrum is often referred to as the "focal plane".
In summary, the tomographic process capitalizes on the angular disparity produced by relative motion between x-ray source and x-ray detector to selectively isolate a region, the location of which can be varied by controlling motion relative to the tissues of interest. In tomographic imaging, linear motion of the source and detectors may be used ("linear tomography") or circular motion of the source and detector may be used ("circular tomography"). Depending upon the diagnostic task, other more complex motions may be used, such as hypercycloidal motion ("hypercycloidal tomography").
A related but computationally distinct imaging process for obtaining tomographic "slices" is computed tomography. In computed tomography, the projection geometry is characterized by a fan-shaped x-ray beam which lies in the same plane as a detector. This geometry renders details in one focal plane independent from those in another focal plane, but at the expense of having the plane of the source and detector motion coincident with the focal plane.
Linear tomography has been used in dental radiography to obtain images of individual teeth or groups of teeth. Computed tomography also has been used in dental radiography to obtain a slice through a subject's head, parallel to the plane of the subject's teeth.
In the above description, and in the description to follow, reference is made to the "plane of the teeth", which is defined as the plane formed by the intersection of a subject's upper and lower teeth. This plane, which is parallel to the plane of the jaw, is typically a horizontal plane when the subject is in a normal upright (standing or sitting) position. Accordingly, the term "horizontal" will be used to refer to a direction parallel to the plane of the teeth, which is typically a horizontal plane. The term "vertical" will be used to refer to a direction perpendicular to the plane of the teeth, which is generally parallel to a subject's spine, and is vertical when the subject is in the typical upright position.
The process of tomography has been extended to digital tomosynthesis, which produces a series of discrete images taken from different positions as the x-ray source and detector move about a fixed fulcrum. Each discrete image corresponds to a different relative position of the source and film plane, rather than a single image produced continuously. In linear tomography and linear tomosynthesis the source and the detector move in a straight line about a fulcrum lying in the tissues which define the plane to be seen sharply in the reconstruction.
It has been shown that the tomosynthetic process is theoretically indistinguishable from the tomographic process, provided that the size of the smallest detail of interest can be specified and the number of discrete projections is sufficiently large. These processes are indistinguishable because they both operate by isolating a focal plane through a controlled blur as described above. However, tomosynthesis has an advantage over tomography, because simple manipulation of the radiographic intensity data obtained from the multiple images permits the position of the focal plane to be adjusted after the fact. Accordingly, a computer can be used to selectively search through the multiple images and to render the image of a particular structure of interest in proper focus, irrespective of its location.
The theoretical and practical designs of a tomosynthetic x-ray system are well known to those having skill in the art, and are described, for example, in an article entitled Tomosynthesis: Three-Dimensional Radiographic Imaging Technique by D. G. Grant, published in the IEEE Transactions on Bio-Medical Engineering, Vol. BME-19, No. 1, Jan., 1972, pp. 20-28. The present inventor likewise has coauthored a number of articles describing computer tomosynthesis. See the articles entitled Computerized Tomosynthesis of Dental Tissues, coauthored with Groenhuis and Ruttimann, published in Oral Surgery, Vol. 56, No. 2, pp. 206-214, Aug., 1983; A Prototype Digital Tomographic X-Ray System For Dental Applications coauthored with Groenhuis and Ruttimann, published in the IEEE International Symposium on Medical Images and Icons, Jul. 24-27, 1984, pp. 218-221; Restoration of Digital Multiplane Tomosynthesis By a Constrained Iteration Method, coauthored with Ruttimann and Groenhuis, published in the IEEE Transactions on Medical Imaging, Vol. MI-3 Sep., 1984, pp. 141-148; and Synthesis of Arbitrary X-ray Projections From a Finite Number of Existing Projections coauthored with Ruttimann, Groenhuis and Edholm, published in Society of Photo-optical Instrumentation of Engineers, Application of Optical Instrumentation in Medicine XIII, Vol. 535, pp. 84-90, 1985.
Panoramic dental radiography is also widely used for dental imaging. In contrast with conventional single tooth exposures, panoramic dental radiography attempts to "unwrap" the curved jaw and teeth into a flat panoramic image. In other words, panoramic dental radiography is a unique imaging technique for showing all of the teeth and related dental tissues on a single exposed film in one "panoramic sweep". It results from a system that dynamically alters the swept projection using an exposure geometry that establishes foci that are different in the horizontal and vertical directions.
Panoramic radiography resembles linear tomography to the extent that it produces images characterized by a region of sharp focus that is controlled by the motion of the x-ray beam and the x-ray film relative to the irradiated tissues. However, unlike tomography the image is produced from a fan-shaped x-ray beam which scans across a moving film as the source of radiation moves in a complicated path around the patient's head. The result is an image which shows relatively sharp images of the teeth unwrapped from the dental arch, but buried in blurred images of other tissues laying further away from the region of interest.
Panoramic radiography involves horizontal rotation of a single fan-shaped x-ray beam and an x-ray detector, such as a photographic plate, about an axis which is generally parallel to the subject's spine and orthogonal to the plane of the teeth. The single fan-shaped beam is a vertical beam, which extends orthogonal to the plane of the teeth. In contrast with tomography, the x-ray film is also moved relative to the x-ray beam, at a constant velocity which is synchronized to the rotation of the x-ray source and film. In the limit (i.e. when the fan-shaped beam is infinitely thin), this movement places the effective tomographic fulcrum at infinity, thus precluding all tomographic blur. On the other hand, by increasing the thickness of the fan-shaped beam, it is possible to introduce more and more tomographic blurring into the resulting panoramic radiograph, to render sharp only the image of structures lying a focal plane determined by the relative movement of the source and detector.
In summary, panoramic radiography sweeps an extended film with a modulated fan-shaped beam to produce an extended image with a lateral extent and projective attributes impossible to produce directly from a single exposure. The degree to which the unwanted out-of-plane structures are tomographically blurred is determined by the horizontal width of the fan beam as it is projected onto the moving film, and by the relative movement of the x-ray source and projected tissues relative to the plane of the fulcrum. Taken together, these processes result in an imaging system that permits the teeth to be viewed tomographically in spite of the fact that they lie in an arched arrangement inside the head.
Panoramic radiography systems are described in U.S Pat. Nos. 4,589,122 to Nieminen entitled Panoramic Tomography X-ray Apparatus; 4,783,793 to Virta et al. entitled X-ray Apparatus for Panoramic Tomography Including Control System; 4,847,881 to Heubeck entitled Dental X-ray Diagnostics Installation For Producing Panoramic Tomograms of the Jaw of a Patient; and 4,878,234 to Pheiffer et al. entitled Dental X-ray Diagnostics Installation for Producing Panorama Slice Exposures of the Jaw of a Patient.
Although useful for producing panoramic images of a subject's teeth, known panoramic dental radiographic systems possess a number of shortcomings. For example, the usefulness of dental panoramic radiography depends greatly on the degree to which the structures of interest lie close to the focal plane. If the region of relative lack of blur is too wide, structures other than teeth and jaws also appear sharp, and obscure details of the structure of diagnostic interest. Hence, in order to prevent unwanted features from appearing clearly, tomographic blurring typically is adjusted to render only images quite near the focal plane in relative focus. Unfortunately, such a design makes the system highly sensitive to subject positioning. If the subject is not positioned properly, the dental structures of interest may not be present in the region of clear tomographic focus, so that the image of important diagnostic details becomes blurred.
A particular problem related to dental panoramic radiography stems from the fact that the images of unwanted structures outside the region of clear focus are only blurred, rather than being eliminated completely. Accordingly, a large diffuse structure is less influenced by the blurring process than a small, sharply delineated tissue detail. This is a particular problem in dental panoramic radiography because the vertical fan-shaped x-ray beam passes through the spine. The midline position of the spine has heretofore precluded x-ray projections which avoid the spine.
The need to pass an x-ray beam through the spine in a dental radiographic applications, produces two deleterious effects. First, it requires a higher x-ray dose than would otherwise be required, in order to provide sufficient x-ray density at the teeth after passing through the spine. Moreover, it produces a large massive blur on the x-ray film which often obscures structures of interest.
Yet another problem with conventional panoramic radiographic systems is the overlap of the teeth in the panoramic radiograph. This overlap takes place because the x-rays are not always directed between the proximal surfaces of the teeth. Accordingly, the interproximal regions of a number of teeth typically overlap in the panoramic radiograph, thereby obscuring necessary details.
It is known that multiple beams can be produced from a single x-ray source, using a suitable x-ray collimator. See U.S. Pat. No. 4,264,824 to Tosswill entitled Far Field Imaging, which describes a collimator for use in imaging far field sources, and U.S. Pat. No. 4,304,999 to Richey et al. entitled Eccentric Source Collimator Assembly for Rotating Source CT Scanner. Multiple beams, which may be produced by collimating a single x-ray source, may be used in multiple beam computer tomography as described in U.S. Pat. No. 4,315,157 to Barnes entitled Multiple Beam Computed Tomography (CT) Scanner. It is also known to use at least two fan-shaped beams having different energy spectra which are obtained from a single x-ray source to produce several negatives or exposures of a single area or zone, and to superpose these exposures, as described in U.S. Pat. No. 4,731,807 to Plessis et al. entitled X-ray Examination Apparatus.
Finally, it is also known to obtain panoramic dental x-ray images from multiple diverging fan-shaped beams, which extend orthogonal to the plane of the teeth, by synchronously rotating the fan-shaped beams, and corresponding detectors, about an axis orthogonal to the plane of the teeth. See U.S. Pat. No. 4,481,650 to Kinanen. Five vertical fan-shaped beams are used, with one beam being a centrally located beam which passes radiation through the subject's spine. The remaining beams are symmetrically arranged about the central beam.
In summary, known panoramic dental radiography techniques, which use single or multiple fan-shaped x-ray beams are highly position sensitive, include excessive overlap of interproximal areas of the teeth, require higher doses, and produce interpretive artifacts due to the effects of the spine.